The present invention relates to the field of networks for distributing electrical energy. In particular, the present invention relates to a method for managing a fault event in a network for distributing low-voltage or high-voltage electrical energy. In a further aspect thereof, the present invention regards an electronic protection unit capable of executing the aforesaid method.
Currently, in networks for distributing low-voltage or high-voltage electrical energy, protection devices are used to which the function of preserving the network from the consequences of a fault event is devolved.
In many cases, said protection devices comprise electromechanical switches of a traditional type in which the function of protection from the fault event is entrusted to the onset of phenomena of electrodynamic repulsion at the level of the electrical closing/opening contacts. Traditionally, the electrical contacts of said switches are, in fact, shaped in such a way that the currents circulating therein flow in mutually opposite directions. The passage of possible currents in the contacts thus determines onset of forces that tend to separate them. In normal operating conditions, said forces of electrodynamic repulsion are, for the most part, of a negligible intensity. In the presence of particularly intense short-circuit currents, said repulsive forces are instead able to complete the separation of the electrical contacts.
Other more advanced protection devices comprise circuit breakers provided with an electronic protection unit for activating, in the event of fault, a kinematic chain that determines separation of the contacts of the circuit breaker.
It is known how, in managing a fault event, the obtaining of a marked tripping selectivity by the aforesaid protection devices assumes particular importance.
By the term “tripping selectivity”, or, more briefly, “selectivity”, is meant the capacity of a system for isolating from the supply relatively circumscribed portions of network in the vicinity of the origin of the fault.
In other words, the term “selectivity” indicates the capacity of tripping in a co-ordinated and circumstantiated way for reducing the lack of service and inconvenience in the event of fault, limiting it just to the portions of network that are effectively at risk and preserving, at the same time, the state of service of the rest of the system.
On the other hand, it is known that obtaining a high level of selectivity is a particularly critical aspect in system design. It is necessary, in fact, to reconcile the requirements of greater tripping reliability with the obvious need to contain the costs of installation and management of the electrical network.
In more traditional electrical networks, a certain level of selectivity is obtained by distributing the protection devices on hierarchical levels differentiated from the energy standpoint (the levels further upstream, i.e., closer to the voltage source, are traditionally considered of higher hierarchy) and selecting the tripping characteristics of each protection device according to the hierarchical level occupied.
Given that each branch of the network is sized to carry, in conditions of safety, electric power sufficient for supplying all the lower levels that propagate therefrom, selectivity is obtained by exploiting the differentiation of the tripping parameters (tripping current, tripping times, mechanical inertia of the contacts, etc.) that exists between protection devices belonging to different hierarchical levels.
In this way, the inconvenience due to a fault event involves only the portion of network regulated by the fault device having a hierarchical level immediately higher than the level or the position in which the fault itself has occurred.
Practice has shown that solutions of this type present certain drawbacks, linked, for the most part, to the fact of not being able to ensure a level of selectivity that is high and constant over time.
In fact, the level of selectivity that can be obtained is strictly correlated to the nominal conditions of operation of the protection devices. It has been shown that, frequently, the performance of the electrical network, in terms of tripping selectivity, can decay considerably, in relation to the state of wear of the components of the protection devices and the consequent variation of the corresponding tripping parameters.
Other more advanced technical solutions of a known type, such as the ones described in the European patents EP838887 and EP856739 and in the U.S. Pat. No. 6,313,639, envisage the use of protection devices, referred to as protection devices of an EFDP (Early Fault Detection Prevention) type, capable of communicating with one another to co-ordinate the respective modalities of tripping, in the case of a fault event.
An EFDP device comprises sensors and corresponding electronic means for acquiring samples of data indicating the instantaneous current and the derivative of the instantaneous current circulating in the equivalent load, corresponding to the portion of network controlled by the device itself.
Said data samples are sent at input to a protection unit and compared with a table of predefined data, corresponding to normal operating conditions. If a number of consecutive data samples do not fall within the set of data corresponding to normal operating conditions, the protection unit of an EFDP device generates an interblock signal.
Said interblock signal is sent to all the other EFDP devices of the network located upstream, causing inhibition of their capacity of tripping.
If the EFDP device in question is not in turn interblocked by another EFDP device set further downstream, it trips directly, thus interrupting the current.
For the above purpose, the protection unit of the EFDP device generates an appropriate opening-command signal, for activating the kinematic chain that determines separation of the electrical contacts.
It is evident how, thanks to the co-ordination of the tripping modalities, in a network comprising EFDP devices levels of selectivity can be obtained that are much higher than those of the more traditional solutions described above.
Albeit meeting the purposes for which they have been devised, also these technical solutions present certain disadvantages.
Said drawbacks basically derive from the fact that obtaining a high selectivity is strictly correlated to the presence of EFDP devices at each node of the network. For the portions of network not presided over by EFDP devices (or, in any case, not communicating with the rest of the electrical network), the level of selectivity remains still linked to the mere differentiation of the tripping parameters, as in more traditional technical solutions.
On the other hand, the use of EFDP devices alone for creating the entire electrical network proves somewhat expensive and laborious.
In order to overcome the above drawbacks, further technical solutions of a known type envisage provision of the EFDP devices installed with a function referred to as “Trip Delay”.
According to said operating mode, after execution of the algorithm described above, an EFDP device waits, for a predefined time interval (for example, 3.5 ms), before intervening directly and interrupting the current.
In this way, it is possible for any other protection device, positioned downstream of the aforesaid EFDP device, to trip, if it is able to do so.
If (and only if) at the end of the aforementioned wait interval, there still exist the fault conditions detected, the EFDP device trips directly, thus interrupting the current.
Is has been noted that, even though this type of solution enables a greater selectivity for more extensive portions of network, it presents the drawback of requiring minimum tripping times (comprised between the 10-20 ms) that are relatively long.
Said tripping times are, at times, unacceptable if it is desired to guarantee a reliable operation over time of the electrical network. For example, if downstream of a certain EFDP device no further EFDP devices are present and the fault occurs at a portion of system comprised between the aforesaid EFDP device and any protection device (not of an EFDP type) set downstream, said portion of system remains exposed to the flow of the fault currents for a time equal to the minimum tripping time of the EFDP device. Given that said fault currents, in particular the short-circuit currents, assume particularly high values (for example, 200 kA), there may occur an early ageing of the parts making up the aforesaid portion of system, jeopardizing proper operation thereof, even in normal conditions.
The U.S. Pat. No. 6,844,737 describes an EFDP device provided with some functionalities aimed at improving the level of selectivity, ensuring, however, relatively reduced tripping times, where necessary.
Said functionalities are based upon the calculation of a quotient between the differential corresponding to two successive samples of current derivative and current, as soon as there has been ascertained a departure from conditions of normality of the evolution of the current in the load.
Practice has, however, shown that said solution, albeit offering a sort of forecast regarding the possible evolution of the fault current, can provide indications that are only partial and not always reliable. For example, for topologically complex portions of network, said method does not make it possible to know whether a protection device downstream of the EFDP is already tripping with a reasonable likelihood of success.